Mechanisms of thermoregulation. The mechanism of thermoregulation of the human body What are the main mechanisms of thermoregulation of the human body

THERMOREGULATION AND HEALTH

The area of human habitation extends from the polar zones, where the air temperature sometimes reaches -86°C, to the equatorial savannas and deserts, in the hottest parts of which it approaches +50°C in the shade! Nevertheless, in such a wide range of temperatures, a person retains active vitality and sufficient performance due to his thermal stability, when body temperature fluctuates within relatively narrow limits - from 36 to 37 ° C.

Homeothermia - constancy of body temperature - makes a person independent of the temperature conditions of residence, since the biochemical reactions that ensure his life continue to be carried out at an optimal level due to the preservation of adequate activity of the tissue enzymes and vitamins that provide them, catalyzing and activating certain aspects of metabolism, tissue hormones, neurotransmitters and other substances on which the normal functioning of the body depends. The shift of temperature in one direction or another sharply changes the activity of these substances, and to a different extent for each of them - as a result, dissociation occurs in the activity of the flow of individual aspects of metabolism. In poikilothermic, cold-blooded animals, whose body temperature is determined by the ambient temperature (increases or decreases along with the latter), the activity of their tissue enzymes as biological catalysts changes along with changes in external thermal conditions. That is why, when the temperature drops, the degree of manifestation of their vital activity decreases up to a complete stop - the so-called suspended animation, and at a very high temperature, either death or drying occurs, which in some of the poikilotherms is also a kind of suspended animation. So, with a change in external temperature, the vital activity of some insects (locusts) can be restored both after freezing to the temperature of liquid nitrogen (–189 ° C) and after drying. A case has been described of the revival, albeit short-term, of a giant newt frozen in a glacier, according to experts, at least about 5000 years ago.

Thus, the ability to maintain a constant body temperature under various conditions of existence makes warm-blooded animals independent of the circumstances of nature and capable of maintaining a high level of viability. This ability is due to a complex system of thermoregulation, which ensures a decrease in heat production and its active return in case of danger of overheating and activation of thermogenesis with limited heat transfer - in case of danger of hypothermia.

Statistics show that in Russia, of all cases of temporary disability, more than 40% are due to colds, which gives reason to the average person to consider the thermoregulation system to be imperfect. However, there are many facts pointing to the high natural resistance of a person to the action of low temperatures. So, yogis compete at temperatures below -20 ° C in the speed of drying wet sheets with the heat of their bodies, sitting naked on the ice of a frozen lake. Swimming by specially trained swimmers across the Bering Strait from Alaska to Chukotka (more than 40 km) at a water temperature of +4°C - +6°C has become traditional. The Yakuts rub newborns with snow, and the Ostyaks and Tunguses immerse them in snow, douse them with cold water and then wrap them in reindeer skins... In this case, apparently, one should rather talk about the perversion of the perfect mechanisms of human thermoregulation by far from the conditions that formed them in evolution life of a modern person than about the imperfection of the mechanisms themselves.

While most of the vital functions - blood circulation, respiration, digestion, etc. - have some specific structural and functional apparatus, thermoregulation does not have such an organ and is a function of the whole organism as a whole.

According to the scheme proposed by I.P. Pavlov, a warm-blooded organism can be represented as a relatively thermostable “core” and a “shell” with a wide temperature range. The core, whose temperature ranges from 36.8–37.5 ° C, includes mainly vital internal organs: the heart, liver, stomach, intestines, etc. Particularly noteworthy is the role of the liver, which has a relatively high temperature - above 37.5 ° C, and the large intestine, the microflora of which, in the course of its life activity, produces a lot of heat, which maintains the temperature of adjacent tissues. The thermolabile shell is made up of limbs, skin and subcutaneous tissues, muscles, etc. The temperature of different sections of the shell varies widely. Thus, the temperature of the toes is about 24°C, the temperature of the ankle joint is 30–31°C, the tip of the nose is 25°C, the armpit, rectum is 36.5–36.9°C, etc. However, the temperature of the shell is very mobile, which is determined by the conditions of vital activity and the state of the body, and therefore its thickness can vary from very thin in heat to very powerful, compressing the core - in cold. Such relationships between the core and the shell are due to the fact that the former predominantly produces heat (at rest), while the latter must ensure the conservation of this heat. This explains the fact that in hardened people, the shell in the cold quickly and reliably envelops the core, maintaining optimal conditions for maintaining the activity of vital organs and systems, while in non-hardened people, the shell remains thin even under these conditions, creating a threat of hypothermia of the core (for example, with a decrease in temperature lungs as little as 0.5°C there is a threat of pneumonia).

The thermal stability of the body is provided mainly by two complementary mechanisms of regulation - physical and chemical. Physical thermoregulation It is mainly activated when there is a danger of overheating and consists in the transfer of heat to the environment. This includes all possible mechanisms of heat transfer: heat radiation, heat transfer, convection and evaporation. Heat radiation is carried out due to infrared rays emanating from the skin that has a high temperature. Heat conduction is realized due to the temperature difference between the skin and the surrounding air. The increase in this difference is due to hyperemia - the expansion of skin vessels and the influx of more warm blood from the internal organs, which is why the color of the skin becomes pink in the heat. At the same time, the efficiency of heat transfer is determined by the thermal conductivity and heat capacity of the external environment: for example, these indicators at the corresponding temperatures for water are 20–27 times higher than for air. From this it becomes clear why the thermocomfortable air temperature for a person is about 18 ° C, and water - 34 ° C. Heat transfer due to evaporation of sweat is very effective, since when 1 ml of sweat evaporates from the surface of the body, the body loses 0.56 kcal of heat. If we take into account that an adult produces about 800 ml of sweat even in conditions of low physical activity, then the effectiveness of this method becomes clear.

Under various conditions of life, the ratio of heat loss in one way or another changes markedly. So, at rest and at optimal air temperature, the body loses 31% of the generated heat by conduction, 44% by radiation, 22% by evaporation (including due to moisture from the respiratory tract) and 3% by convection. With a strong wind, the role of convection increases, with an increase in air humidity - conduction, and with increased work - evaporation (for example, with intense physical activity, evaporation of sweat sometimes reaches 3-4 liters per hour!).

The body's heat transfer efficiency is exceptionally high. Biophysical calculations show that a violation of these mechanisms, even in a person at rest, would lead to an increase in his body temperature within an hour up to 37.5 ° C, and after 6 hours - up to 46-48 ° C, when the irreversible destruction of protein structures begins.

Chemical thermoregulation is of particular importance when there is a danger of hypothermia. The loss of a woolen cover by a person relative to animals made him especially sensitive to the action of low temperatures, as evidenced by the fact that a person has almost 30 times more cold receptors than heat receptors. At the same time, the improvement of the mechanisms of adaptation to cold has led to the fact that a person tolerates a decrease in body temperature much more easily than its increase. Thus, infants easily tolerate a decrease in body temperature by 3–5 ° C, but it is difficult to tolerate an increase of 1–2 ° C. An adult without any consequences tolerates hypothermia up to 33–34 ° C, but loses consciousness when overheated from external sources up to 38.6 ° C, although with a fever from an infection, he can retain consciousness even at 42 ° C. At the same time, cases of revival of frozen people, whose skin temperature fell below the freezing point, were noted.

The essence of chemical thermoregulation is to change the activity of metabolic processes in the body: at a high external temperature, it decreases, and at a low one, it increases. Studies show that with a decrease in ambient temperature by 1 ° C in a naked person at rest, metabolic activity increases by 10%. (However, anesthesia and the so-called neuroleptics turn off the higher regulatory mechanisms of thermal stability in warm-blooded animals makes them dependent on the ambient temperature, and when their body temperature is cooled to 32 ° C, their oxygen consumption decreases to 50%, at 20 ° C - to 20%, and when +1°С – up to 1% of the initial level.)

Of particular importance for maintaining body temperature is the tone of skeletal muscles, which increases with a decrease in ambient temperature and decreases with warming. It is significant that these processes proceed the more actively, the more dangerous the threatening violation of thermal stability. Thus, at an air temperature of 25–28°C (and especially in combination with high humidity), the muscles are largely relaxed, and the thermal energy reproduced by them is negligible. On the contrary, with the danger of hypothermia, trembling becomes more and more important - uncoordinated contractions of muscle fibers, when external mechanical work is almost completely absent, and almost all the energy of contracting fibers is converted into thermal energy (this phenomenon is called non-contractile thermogenesis). There is nothing surprising, therefore, in the fact that during shivering the body's heat production can increase by more than three times, and during strenuous physical work - by 10 or more times.

The lungs also play an undoubted role in chemical thermoregulation, which, due to changes in the metabolic activity of the high-calorie fats included in their structure, maintain a relatively constant temperature, which is why at a high external temperature the blood flowing from the lungs is cooler, and at a low temperature it is warmer than inhaled air.

The physical and chemical mechanisms of thermoregulation work with a high degree of coordination due to the presence in the central nervous system of the corresponding center in the diencephalon (hypothalamus). That is why, at high ambient temperatures, on the one hand, heat transfer increases (due to an increase in skin temperature, evaporation of sweat, etc.), and on the other hand, heat production decreases (due to a decrease in muscle tone, the transition to the absorption of less energy-containing products by the body); at low temperatures, on the contrary: heat production increases and heat transfer decreases.

Thus, the perfect mechanisms of human thermoregulation allow maintaining optimal viability in a wide range of external temperatures.

The body temperature of humans and higher animals is maintained at a relatively constant level, despite fluctuations in ambient temperature. This constant body temperature is called isotherms.

Isotherm is characteristic only of the so-called homoiothermic, or warm-blooded, animals and absent in poikilothermic, or cold-blooded, animals whose body temperature is variable and differs little from the ambient temperature.

Isothermy in the process of ontogenesis develops gradually. In a newborn baby, the ability to maintain a constant body temperature is far from perfect. As a result, cooling may occur. (hypothermia) or overheating (hyperthermia) body at ambient temperatures that do not affect an adult. Likewise, even a small amount of muscular work, such as prolonged crying of a child, can lead to an increase in body temperature. The body of premature babies is even less able to maintain a constant body temperature, which in them largely depends on the temperature of the environment.

Heat generation occurs as a result of continuously occurring exothermic reactions. These reactions occur in all organs and tissues, but with different intensity. In tissues and organs that perform active work - in muscle tissue, liver, kidneys - more heat is released than in less active ones - connective tissue, bones, cartilage.

Heat loss by organs and tissues depends to a large extent on their location: superficially located organs, such as skin, skeletal muscles, give off more heat and cool more strongly than internal organs, which are more protected from cooling.

The body temperature of a healthy person is 36.5-36.9 °C. Rest and sleep are lowered, and muscle activity raises body temperature. The maximum temperature is observed at 16-18 pm, the minimum - at 3-4 am. For workers who work long night shifts, temperature fluctuations can be reversed.

The constancy of body temperature in a person can be maintained only if the heat generation and heat loss of the whole organism are equal. This is achieved through the physiological mechanisms of thermoregulation. manifests itself as a result of the interaction of the processes of heat generation and heat transfer, regulated by neuroendocrine mechanisms. Thermoregulation is usually divided into chemical and physical.

Chemical thermoregulation carried out by changing the level of heat generation, i.e. strengthening or weakening the intensity of metabolism in the cells of the body, and is important for maintaining a constant body temperature both under normal conditions and when the ambient temperature changes.

The most intense heat generation in the body occurs in the muscles. Even if a person lies motionless, but his muscles are tense, the intensity of oxidative processes, and at the same time heat generation, increase by 10%. A small physical activity leads to an increase in heat generation by 50-80%, and heavy muscular work - by 400-500%.

In cold conditions, heat generation in the muscles increases, even if the person is stationary. This is due to the fact that the cooling of the surface of the body, acting on receptors that perceive cold irritation, reflexively excites chaotic involuntary muscle contractions, manifested in the form of trembling (chills). At the same time, the metabolic processes of the body are significantly enhanced, the consumption of oxygen and carbohydrates by muscle tissue increases, which entails an increase in heat generation. Even arbitrary shaking increases heat generation by 200%. If muscle relaxants are introduced into the body - substances that disrupt the transmission of nerve impulses from the nerve to the muscle and thereby eliminate reflex muscle tremors, even with an increase in ambient temperature, a decrease in body temperature occurs much faster.

The liver and kidneys also play a significant role in chemical thermoregulation. The blood temperature of the hepatic vein is higher than the blood temperature of the hepatic artery, which indicates intense heat generation in this organ. When the body is cooled, heat production in the liver increases.

The release of energy in the body occurs due to the oxidative breakdown of proteins, fats and carbohydrates; therefore, all the mechanisms that regulate oxidative processes also regulate heat generation.

Physical thermoregulation carried out by changes in the release of heat by the body. It acquires particular importance in maintaining a constant body temperature during the stay of the body in conditions of elevated ambient temperature.

Heat transfer is carried out by heat radiation (radiative heat transfer), or convection, those. movement and movement of heated air, heat conduction, those. heat transfer to substances in direct contact with the surface of the body, and water evaporation from the surface of the skin and lungs.

In humans, under normal conditions, heat loss by conduction is small, since air and clothing are poor conductors of heat. Radiation, evaporation and convection proceed with different intensity depending on the ambient temperature. In a person at rest at an air temperature of about 20 ° C and a total heat transfer equal to 419 kJ (100 kcal) per hour, 66% is lost with the help of radiation, 19% due to water evaporation, and 15% of the total body heat loss due to convection . When the ambient temperature rises to 35 ° C, heat transfer with the help of radiation and convection becomes impossible and the body temperature is maintained at a constant level solely by the evaporation of water from the surface of the skin and the alveoli of the lungs.

Clothing reduces heat transfer. Heat loss is prevented by the layer of still air that is between clothing and skin, since air is a poor conductor of heat. The heat-insulating properties of clothing are the higher, the finer the cellular structure of its structure, which contains air. This explains the good heat-insulating properties of woolen and fur clothing. The air temperature under the clothes is 30°C. On the contrary, a naked body loses heat, as the air on its surface is constantly being replaced. Therefore, the temperature of the skin of the naked parts of the body is much lower than that of the dressed ones.

In the cold, the blood vessels of the skin, mainly arterioles, narrow: more blood enters the vessels of the abdominal cavity, and thus heat transfer is limited. The surface layers of the skin, receiving less warm blood, radiate less heat - heat transfer decreases. With a strong cooling of the skin, in addition, the opening of arteriovenous anastomoses occurs, which reduces the amount of blood entering the capillaries, and thereby prevents heat transfer.

The redistribution of blood that occurs in the cold - a decrease in the amount of blood circulating through the superficial vessels, and an increase in the amount of blood passing through the vessels of the internal organs - contributes to the preservation of heat in the internal organs.

When the ambient temperature rises, the vessels of the skin expand, the amount of blood circulating in them increases. The volume of circulating blood throughout the body also increases due to the transfer of water from the tissues to the vessels, and also because the spleen and other blood depots throw additional blood into the general circulation. Increasing the amount of blood circulating through the surface vessels of the body promotes heat transfer through radiation and convection.

To maintain a constant human body temperature at a high ambient temperature, evaporation of sweat from the skin surface is of primary importance, which depends on the relative humidity of the air. In air saturated with water vapor, water cannot evaporate. Therefore, at high humidity of atmospheric air, high temperature is more difficult to tolerate than at low humidity. In the air saturated with water vapor (for example, in a bath), sweat is released in large quantities, but does not evaporate and drains from the skin. Such sweating does not contribute to the release of heat: only that part of the sweat that evaporates from the surface of the skin is important for heat transfer (this part of the sweat is called effective perspiration).

Clothing impervious to air (rubber, etc.), which prevents the evaporation of sweat, is poorly tolerated: the layer of air between clothing and the body is quickly saturated with vapor and further evaporation of sweat stops.

Since some of the water is evaporated by the lungs in the form of vapors that saturate the exhaled air, breathing also participates in maintaining body temperature at a constant level. At a high ambient temperature, the respiratory center is reflexively excited, at a low temperature it is depressed, breathing becomes less deep.

Thus, the constancy of body temperature is maintained through the joint action, on the one hand, of the mechanisms that regulate the intensity of metabolism and the heat generation that depends on it (chemical regulation of heat), and on the other hand, the mechanisms that regulate heat transfer (physical regulation of heat) (Fig. 9.10) .

Rice. 9.10.

Isothermal regulation. Regulatory reactions that maintain a constant body temperature are complex reflex acts that occur in response to thermal stimulation of skin receptors, skin and subcutaneous vessels, as well as the central nervous system itself. These receptors that perceive cold and heat are called thermoreceptors. At a relatively constant ambient temperature, rhythmic impulses arrive from the receptors in the central nervous system, reflecting their tonic activity. The frequency of these impulses is maximum for cold receptors of the skin and skin vessels at a temperature of 20-30 °C, and for skin heat receptors - at a temperature of 38-43 °C. With a sharp cooling of the skin, the frequency of impulses in cold receptors increases, and with rapid warming it becomes less or stops. Thermal receptors react to the same temperature drops in the opposite way. Thermal and cold receptors of the central nervous system respond to changes in the temperature of the blood flowing to the nerve centers (central thermoreceptors). The main part of the heat is generated by the skeletal muscles and internal organs, which form the core, and the skin creates a shell aimed at retaining or removing heat from the body (Fig. 9.11).

Rice. 9.11.

The hypothalamus contains the main thermoregulation centers, which coordinate numerous and complex processes that ensure the preservation of body temperature at a constant level. This is proved by the fact that the destruction of the hypothalamus entails the loss of the ability to regulate body temperature and makes the animal poikilothermic, while the removal of the cerebral cortex, striatum and optic tubercles does not noticeably affect the processes of heat generation and heat transfer.

In the implementation of the hypothalamic regulation of body temperature, the endocrine glands, mainly the thyroid and adrenal glands, are involved.

The participation of the thyroid gland in thermoregulation is proved by the fact that the introduction into the blood of an animal of the blood serum of another animal, which has been in the cold for a long time, causes an increase in metabolism in the former. This effect is observed only when the thyroid gland is preserved in the second animal. Obviously, during a stay in conditions of cooling, there is an increased release into the blood of the thyroid hormone, which increases metabolism and, consequently, the formation of heat.

The participation of the adrenal glands in thermoregulation is due to the release of adrenaline into the blood, which, by enhancing oxidative processes in tissues, in particular in muscles, increases heat generation and constricts skin vessels, reducing heat transfer. Therefore, adrenaline can cause an increase in body temperature ( adrenaline hyperthermia).

Hypothermia and hyperthermia. If a person is in conditions of significantly increased or decreased ambient temperature for a long time, then the mechanisms of physical and chemical thermoregulation of heat, due to which, under normal conditions, the body temperature is maintained constant, may be insufficient: hypothermia of the body occurs or overheating - hyperthermia.

Hypothermia - a state in which body temperature drops below 35 ° C. Hypothermia occurs most quickly when immersed in cold water. In this case, excitation of the sympathetic nervous system is observed first, heat transfer is reflexively limited and heat production is enhanced. The latter is facilitated by muscle contraction - muscle tremors. After a while, the body temperature still begins to decrease. In this case, a state similar to anesthesia is observed: the disappearance of sensitivity, the weakening of reflex reactions, and the decrease in the excitability of the nerve centers. The intensity of metabolism sharply decreases, breathing slows down, heart contractions slow down, cardiac output decreases, blood pressure decreases (at a body temperature of 24-25 ° C, it can be 15-20% of the original).

In recent years, artificially created hypothermia with body cooling to 24–28 °C has been used in surgical clinics that perform heart and central nervous system operations. The meaning of this event is that hypothermia significantly reduces the metabolism of the brain and, consequently, the need for oxygen in this organ. As a result, a longer bleeding of the brain becomes possible (instead of 3-5 minutes at normal temperature to 15-20 minutes at 25-28 ° C), which means that during hypothermia, patients more easily tolerate temporary shutdown of cardiac activity and respiratory arrest.

Cryotherapy is also used for some other diseases.

Hyperthermia - a state in which body temperature rises above 37 ° C. It occurs during prolonged exposure to high ambient temperatures, especially when the air is humid and therefore there is little effective perspiration. Hyperthermia can also occur under the influence of some endogenous factors that increase heat generation in the body (thyroxine, fatty acids, etc.). Sharp hyperthermia, at which body temperature reaches 40-41 ° C, is accompanied by a severe general condition of the body and is called heat stroke.

Such a change in temperature should be distinguished from hyperthermia, when the external conditions are not changed, but the actual process of thermoregulation is violated. An example of such a disorder is infectious fever. One of the reasons for its occurrence is the high sensitivity of the hypothalamic centers for the regulation of heat transfer to certain chemical compounds, in particular to bacterial toxins.

Thus, the balance of factors responsible for heat production and heat transfer is the main mechanism of thermoregulation.

Questions and tasks

1. What is the role of proteins in the body? What is the essence of the regulation of protein metabolism?
2. What is the role of carbohydrates in the body? What is the essence of the regulation of carbohydrate metabolism?
3. What is the role of fats in the body? What is the essence of the regulation of fat metabolism?
4. What is the importance of vitamins in human life?
5. The value of physical and chemical thermoregulation in the body. Explain the answer.
6. In recent years, artificially created hypothermia with body cooling to 24-28 °C has been used in practice in surgical clinics performing heart and central nervous system operations. What is the meaning of this event?

In warm-blooded animals and humans (the so-called homoiothermic organisms), in contrast to cold-blooded (or poikilothermic) ones, a constant body temperature is a prerequisite for existence, one of the cardinal parameters of homeostasis (or constancy) of the internal environment of the body.

Physiological mechanisms that provide thermal homeostasis of the body (its "core") are divided into two functional groups: the mechanisms of chemical and physical thermoregulation. Chemical thermoregulation is the regulation of body heat production. Heat is constantly produced in the body in the process of redox reactions of metabolism. At the same time, part of it is given to the external environment the more, the greater the difference between the temperature of the body and the environment. Therefore, maintaining a stable body temperature with a decrease in environmental temperature requires a corresponding increase in metabolic processes and the accompanying heat generation, which compensates for heat loss and leads to the preservation of the overall heat balance of the body and maintaining a constant internal temperature. The process of reflex enhancement of heat production in response to a decrease in ambient temperature is called chemical thermoregulation. The release of energy in the form of heat accompanies the functional load of all organs and tissues and is characteristic of all living organisms. The specificity of the human body is that the change in heat production as a reaction to changing temperature is a special reaction of the body that does not affect the level of functioning of the main physiological systems.

Specific thermoregulatory heat generation is concentrated mainly in the skeletal muscles and is associated with special forms of muscle functioning that do not affect their direct motor activity. An increase in heat generation during cooling can also occur in a resting muscle, as well as when the contractile function is artificially turned off by the action of specific poisons.

One of the most common mechanisms of specific thermoregulatory heat generation in muscles is the so-called thermoregulatory tone. It is expressed by microcontractions of fibrils, recorded as an increase in the electrical activity of an externally immobile muscle during its cooling. Thermoregulatory tone increases oxygen consumption by the muscle, sometimes by more than 150%. With stronger cooling, along with a sharp increase in thermoregulatory tone, visible muscle contractions in the form of cold shivering are included. Gas exchange in this case increases up to 300-400%. Characteristically, the muscles are unequal in terms of the share of participation in thermoregulatory heat generation.

With prolonged exposure to cold, the contractile type of thermogenesis can be replaced (or supplemented) to one degree or another by switching tissue respiration in the muscle to the so-called free (non-phosphorylating) pathway, in which the phase of formation and subsequent breakdown of ATP falls out. This mechanism is not associated with the contractile activity of the muscles. The total mass of heat released during free respiration is practically the same as during yeast thermogenesis, but most of the heat energy is consumed immediately, and oxidative processes cannot be inhibited by a lack of ADP or inorganic phosphate.

The latter circumstance makes it possible to freely maintain a high level of heat generation for a long time.

Changes in the intensity of metabolism caused by the influence of environmental temperature on the human body are natural. In a certain range of external temperatures, heat production corresponding to the exchange of a resting organism is completely compensated by its “normal” (without active intensification) heat transfer. The heat exchange of the body with the environment is balanced. This temperature range is called the thermoneutral zone. The level of exchange in this zone is minimal. Often they speak of a critical point, implying a specific temperature value at which a thermal balance with the environment is achieved. Theoretically, this is true, but it is practically impossible to establish such a point experimentally due to constant irregular fluctuations in metabolism and the instability of the heat-insulating properties of the covers.

A decrease in the temperature of the environment outside the thermoneutral zone causes a reflex increase in the level of metabolism and heat production until the body's heat balance is balanced under new conditions. Because of this, the body temperature remains unchanged.

An increase in the temperature of the environment outside the thermoneutral zone also causes an increase in the level of metabolism, which is caused by the activation of mechanisms for activating heat transfer, requiring additional energy costs for their work. This forms a zone of physical thermoregulation, during which the temperature also remains stable. Upon reaching a certain threshold, the mechanisms for enhancing heat transfer turn out to be ineffective, overheating begins and, finally, the death of the organism.

Back in 1902, Rubner proposed to distinguish between two types of these mechanisms - "chemical" and "physical" thermoregulation. The first is associated with a change in heat production in tissues (the voltage of chemical reactions of exchange), the second is characterized by heat transfer and redistribution of heat. Along with blood circulation, an important role in physical thermoregulation belongs to perspiration, therefore, a special function of heat transfer belongs to the skin - here the blood heated in the muscles or in the "core" cools down, the mechanisms of perspiration and perspiration are realized here.

b In the "normal" heat conduction can be neglected, because the thermal conductivity of air is low. The thermal conductivity of water is 20 times higher, so heat transfer by conduction plays a significant role and becomes a significant factor in hypothermia in the case of wet clothes, damp socks, etc.

b More efficient heat transfer by convection (i.e., the movement of gas or liquid particles, mixing their heated layers with cooled ones). In an air environment, even at rest, heat transfer by convection accounts for up to 30% of heat loss. The role of convection in the wind or in the movement of a person increases even more.

b The transfer of heat by radiation from a heated body to a cold one takes place according to the Stefan-Boltzmann law and is proportional to the difference in the fourth degrees of temperature of the skin (clothes) and the surface of surrounding objects. In this way, under "comfort" conditions, a naked person gives up up to 45% of thermal energy, but for a warmly dressed person, heat loss by radiation does not play a special role.

b Evaporation of moisture from the skin and the surface of the lungs is also an effective way of heat transfer (up to 25%) under "comfort" conditions. Under conditions of high ambient temperature and intense muscular activity, heat transfer by sweat evaporation plays a dominant role - 0.6 kcal of energy is carried away with 1 gram of sweat. It is easy to calculate the total amount of heat lost with sweat, given that under conditions of intense muscular activity, a person can give up to 10-12 liters of fluid in an eight-hour working day. In the cold, heat loss through sweat in a well-dressed person is small, but even here one must take into account heat transfer due to breathing. In this process, two mechanisms of heat transfer are combined at once - convection and evaporation. The loss of heat and fluid with respiration is quite significant, especially during intense muscular activity in conditions of low atmospheric humidity.

A significant factor influencing the processes of thermoregulation are vasomotor (vasomotor) reactions of the skin. With the most pronounced narrowing of the vascular bed, heat loss can decrease by 70%, with maximum expansion - increase by 90%.

Specific differences in chemical thermoregulation are expressed in the difference in the level of the main (in the zone of thermoneutrality) metabolism, the position and width of the thermoneutral zone, the intensity of chemical thermoregulation (an increase in metabolism with a decrease in ambient temperature by 1 "C), as well as in the range of effective thermoregulation. All these parameters reflect ecological specifics of individual species and adaptively change depending on the geographical location of the region, the season of the year, altitude and a number of other environmental factors.

Regulatory responses aimed at maintaining a constant body temperature during overheating are represented by various mechanisms for enhancing heat transfer to the external environment. Among them, heat transfer is widespread and has a high efficiency by intensifying the evaporation of moisture from the surface of the body and (and) the upper respiratory tract. When moisture evaporates, heat is consumed, which can contribute to maintaining the heat balance. The reaction is turned on when there are signs of an incipient overheating of the body.

So, adaptive changes in heat transfer in the human body can be aimed not only at maintaining a high level of metabolism, as in most people, but also at setting a low level in conditions that threaten to deplete energy reserves.

Thermoregulation is associated with the mechanisms of regulation of the level of heat production (chemical regulation) and heat transfer (physical regulation). The balance of heat production and heat transfer is controlled by the hypothalamus, which integrates sensory, vegetative, emotional, and motor components of adaptive behavior.

The perception of temperature is carried out by receptor formations on the surface of the body (skin receptors) and deep temperature receptors in the respiratory tract, blood vessels, internal organs, and in the intermuscular nerve plexuses of the gastrointestinal tract. Afferent nerves send impulses from these receptors to the thermoregulatory center in the hypothalamus. It activates various mechanisms that provide either heat production or heat transfer. The feedback mechanism involving the nervous system and blood flow change the sensitivity of temperature receptors (Fig. 15.4, 15.5). Thermosensitive formations are also located in different areas of the central nervous system - in the motor cortex, in the hypothalamus, in the region of the brain stem (reticular formation, medulla oblongata) and spinal cord.

In the hypothalamus, which is sometimes called the "thermostat of the body", there is not only a center that integrates various sensory impulses associated with information about heat

Rice. 15.4.

balance of the body, but also the center of regulation of motor reactions that control changes in the temperature regime. After dysfunction of the hypothalamus, the ability to regulate body temperature is lost.

The control of the regulation of heat transfer to prevent overheating is associated with the anterior hypothalamus - its neurons are sensitive to the temperature of the flowing blood. In case of disruption of the work of this center, control over body temperature is maintained in a cold environment, but in the heat it is absent and the body temperature rises significantly.

Another thermoregulatory center located in the posterior hypothalamus controls the amount of heat production.

Rice. 15.5. The participation of the nervous system in thermoregulation and thus prevents excessive cooling. Violation of the work of this center reduces the ability to increase energy metabolism in a cold environment, and the body temperature drops.

The transfer of heat from the internal regions of the body to the extremities as a result of changes in the volume of blood flow is an important means of regulating heat transfer through vasomotor reactions. The limbs withstand a much wider range of temperatures than the internal regions of the body, and form excellent thermal "vents", i.e. places that can provide more or less heat loss, depending on the influx of heat from the internal regions of the body through the bloodstream.

Thermoregulation is associated with the sympathetic nervous system (see Fig. 15.5). It regulates vascular tone; as a result, blood flow to the skin changes (see Chapter 4). The expansion of the subcutaneous vessels is accompanied by a slowdown in blood flow in them and an increase in heat transfer (Fig. 15.6). In extreme heat, blood flow to the skin of the extremities increases dramatically, and excess heat is dissipated. The proximity of the veins to the skin surface increases the cooling of the blood, which returns to the internal regions of the body.

When cooled, the vessels narrow, and blood flow to the periphery decreases. In humans, as blood passes through the large vessels of the hands and yoga, its temperature drops. The cooled venous blood, returning inside the body through the vessels located near the arteries, captures a large

Rice. 15.6. The reaction of the superficial vessels of the skin to cold - narrowing (a) and heat - expansion (b)

the proportion of heat given off by the arterial blood. Such a system is called countercurrent heat exchange. It promotes the return of a large amount of heat to the internal areas of the body after the passage of blood through the limbs. The overall effect of such a system is a reduction in heat transfer. At an air temperature close to zero, such a system is not beneficial, since as a result of intense heat exchange between arterial and venous blood, the temperature of the fingers and toes can significantly decrease, which can cause frostbite.

The main source of heat production is associated with muscle contractions, which are under voluntary control. Another type of increased heat production in the body can be muscle tremors - a reaction to cold. A slight movement of the muscles during shivering increases the efficiency of heat production. When trembling, the flexors and extensors of the limbs and the masticatory muscles contract rhythmically and simultaneously with great frequency. The frequency and strength of the contraction may vary. Trembling is only generated if said muscles are not involved in another activity. It can be overcome by voluntary muscular work. Voluntary movements, such as walking, are associated with muscle contraction that overcomes trembling. Both trembling and walking are accompanied by the formation of heat. Neurons of the posterior hypothalamus influence the frequency and strength of muscle contractions during trembling. This center receives impulses from the thermoregulatory center in the anterior hypothalamus and from muscle receptors. Impulses from the brain come to all levels of the spinal cord, where rhythmic signals arise that cause trembling in the muscles.

In addition, thermal energy is generated by the breakdown of fats stored in adipose tissue. The most effective in this sense is brown fat, located in newborns between the shoulder blades and behind the sternum. Within a few days after birth, heat production, which is provided by brown fat cells, is the main reaction to cold. Later in children, this reaction becomes trembling. Brown fat is found in large quantities in hibernating animals. The breakdown of fat from white adipose tissue is less efficient. White fat does not contribute to the formation, but to the preservation of heat.

A. Human life can proceed only in a narrow range of temperatures.

Temperature has a significant impact on the course of life processes in the human body and on its physiological activity. Life processes are limited by a narrow temperature range of the internal environment, in which the main enzymatic reactions can occur. For humans, a decrease in body temperature below 25°C and an increase above 43°C is usually fatal. Nerve cells are especially sensitive to changes in temperature.

Heat causes intense sweating, which leads to dehydration, loss of mineral salts and water-soluble vitamins. The consequence of these processes is blood clotting, impaired salt metabolism, gastric secretion, and the development of vitamin deficiency. The allowable reduction in weight during evaporation is 2-3%. With a weight loss from evaporation of 6%, mental activity is disturbed, and with 15-20% weight loss, death occurs. The systematic effect of high temperature causes changes in the cardiovascular system: increased heart rate, changes in blood pressure, weakening of the functional capacity of the heart. Prolonged exposure to high temperature leads to the accumulation of heat in the body, while the body temperature may rise to 38-41 ° C and heat stroke with loss of consciousness may occur.

Low temperatures can be the causes of cooling and hypothermia of the body. When cooling in the body, heat transfer decreases reflexively and heat production increases. A decrease in heat transfer occurs due to spasm (narrowing) of blood vessels, an increase in thermal resistance of body tissues. Prolonged exposure to low temperature leads to persistent vascular spasm, tissue malnutrition. The growth of heat production during cooling is achieved by the effort of oxidative metabolic processes in the body (a decrease in body temperature by 1°C is accompanied by an increase in metabolic processes by 10°C). Exposure to low temperatures is accompanied by an increase in blood pressure, inspiratory volume and a decrease in respiratory rate. Cooling the body changes carbohydrate metabolism. Great cooling is accompanied by a decrease in body temperature, inhibition of the functions of organs and body systems.

B. The core and the outer shell of the body.

From the point of view of thermoregulation, the human body can be represented as consisting of two components - external shells and internal nuclei.

Core- this is a part of the body that has a constant temperature (internal organs), and shell- a part of the body in which there is a temperature gradient (these are tissues of the surface layer of the body with a thickness of 2.5 cm). Through the shell, there is a heat exchange between the core and the environment, that is, changes in the thermal conductivity of the shell determine the constancy of the temperature of the core. Thermal conductivity changes due to changes in blood supply and blood supply to the tissues of the shell.

The temperature of different parts of the core is different. For example, in the liver: 37.8-38.0°C, in the brain: 36.9-37.8°C. In general, the core temperature of the human body is 37.0°С. This is achieved through the processes of endogenous thermoregulation, the result of which is a stable balance between the amount of heat produced in the body per unit time ( heat production) and the amount of heat dissipated by the body during the same time into the environment ( heat dissipation).

The temperature of human skin in different areas ranges from 24.4°C to 34.4°C. The lowest temperature is observed on the toes, the highest - in the armpit. It is on the basis of measuring the temperature in the armpit that the body temperature at a given time is usually judged.

According to averaged data, the average skin temperature of a naked person in conditions of comfortable air temperature is 33-34°C. There are daily fluctuations in body temperature. The oscillation amplitude can reach 1°C. Body temperature is minimal in the early morning hours (3-4 hours) and maximum in the daytime (16-18 hours).

The phenomenon of temperature asymmetry is also known. It is observed in about 54% of cases, and the temperature in the left armpit is slightly higher than in the right. Asymmetry is also possible in other areas of the skin, and the severity of asymmetry of more than 0.5 ° C indicates pathology.

B. Heat transfer. The balance of heat generation and heat transfer in the human body.

The processes of human vital activity are accompanied by continuous heat generation in his body and the release of the generated heat into the environment. The exchange of thermal energy between the body and the environment is called p heat exchange. Heat production and heat transfer are due to the activity of the central nervous system, which regulates metabolism, blood circulation, sweating and the activity of skeletal muscles.

The human body is a self-regulating system with an internal heat source, in which, under normal conditions, heat production (the amount of heat generated) is equal to the amount of heat given off to the external environment (heat transfer). The constancy of body temperature is called isotherm. It ensures the independence of metabolic processes in tissues and organs from fluctuations in ambient temperature.

The internal temperature of the human body is constant (36.5-37°C) due to the regulation of the intensity of heat production and heat transfer depending on the temperature of the external environment. And the temperature of human skin under the influence of external conditions can vary within relatively wide limits.

In the human body in 1 hour, as much heat is generated as needed to boil 1 liter of ice water. And if the body were a case impervious to heat, then in an hour the body temperature would rise by about 1.5 ° C, and after 40 hours it would reach the boiling point of water. During hard physical work, heat generation increases several times more. Yet our body temperature does not change. Why? It's all about balancing the processes of formation and release of heat in the body.

The leading factor determining the level of heat balance is ambient temperature. When it deviates from the comfort zone, a new level of heat balance is established in the body, which ensures isotherm under new environmental conditions. This constancy of body temperature is provided by the mechanism thermoregulation, including the process of heat generation and the process of heat release, which are regulated by the neuro-endocrine pathway.

D. The concept of body thermoregulation.

thermoregulation- this is a set of physiological processes aimed at maintaining a relative constancy of the temperature of the core of the body in conditions of changing environmental temperature through the regulation of heat production and heat transfer. Thermoregulation is aimed at preventing violations of the body's thermal balance or at its restoration, if such violations have already occurred, and is carried out by the neuro-humoral way.

It is generally accepted that thermoregulation is characteristic only of homoiothermic animals (these include mammals (including humans) and birds), whose body has the ability to maintain the temperature of the internal regions of the body at a relatively constant and fairly high level (about 37-38 ° C in mammals). and 40-42°C in birds) regardless of changes in ambient temperature.

The thermoregulation mechanism can be represented as a cybernetic self-control system with feedback. Temperature fluctuations of the surrounding air act on special receptor formations ( thermoreceptors) are temperature sensitive. Thermoreceptors transmit information about the thermal state of the organ to the thermoregulation centers, in turn, the thermoregulation centers through nerve fibers, hormones and other biologically active substances change the level of heat transfer and heat production or parts of the body (local thermoregulation), or the body as a whole. When the thermoregulation centers are turned off by special chemicals, the body loses the ability to maintain a constant temperature. In recent years, this feature has been used in medicine for artificial cooling of the body during complex surgical operations on the heart.

Skin thermoreceptors.

It is estimated that a person has about 150,000 cold and 16,000 heat receptors that respond to changes in the temperature of the internal organs. Thermoreceptors are located in the skin, internal organs, respiratory tract, skeletal muscles and the central nervous system.

Thermoreceptors of the skin are rapidly adapting and react not so much to the temperature itself, but to its changes. The maximum number of receptors is located in the head and neck, the minimum - on the limbs.

Cold receptors are less sensitive and their threshold of sensitivity is 0.012°C (when cooled). The sensitivity threshold of thermal receptors is higher and amounts to 0.007°C. This is probably due to the greater danger to the body of overheating.

D. Types of thermoregulation.

Thermoregulation can be divided into two main types:

1. Physical thermoregulation:

Evaporation (sweating);

Radiation (radiation);

Convection.

2. Chemical thermoregulation.

Contractile thermogenesis;

non-shivering thermogenesis.

Physical thermoregulation(a process that removes heat from the body) - ensures the preservation of body temperature by changing the body's heat transfer through the skin (conduction and convection), radiation (radiation) and water evaporation. The return of heat constantly generated in the body is regulated by changes in the thermal conductivity of the skin, subcutaneous fat layer and epidermis. Heat transfer is largely regulated by the dynamics of blood circulation in heat-conducting and heat-insulating tissues. As the ambient temperature rises, evaporation begins to dominate heat transfer.

Conduction, convection and radiation are passive heat transfer paths based on the laws of physics. They are effective only if a positive temperature gradient is maintained. The smaller the temperature difference between the body and the environment, the less heat is given off. With the same indicators or at a high ambient temperature, the mentioned ways are not only ineffective, but the body also heats up. Under these conditions, only one mechanism of heat transfer is triggered in the body - sweating.

At low ambient temperatures (15°C and below), about 90% of daily heat transfer occurs due to heat conduction and heat radiation. Under these conditions, no visible sweating occurs. At an air temperature of 18-22°C, heat transfer due to thermal conductivity and heat radiation decreases, but heat loss by the body increases by evaporating moisture from the skin surface. When the ambient temperature rises to 35 ° C, heat transfer using radiation and convection becomes impossible, and the body temperature is maintained at a constant level solely by the evaporation of water from the surface of the skin and the alveoli of the lungs. With high humidity, when the evaporation of water is difficult, overheating of the body may occur and heat stroke may develop.

In a person at rest at an air temperature of about 20 ° C and a total heat transfer equal to 419 kJ (100 kcal) per hour, 66% is lost with the help of radiation, 19% of water evaporation, and 15% of the total body heat loss by convection.

Chemical thermoregulation(the process that ensures the formation of heat in the body) - is realized through the metabolism and through the heat production of tissues such as muscles, as well as the liver, brown fat, that is, by changing the level of heat generation - by increasing or weakening the intensity of metabolism in the cells of the body. When organic matter is oxidized, energy is released. Part of the energy goes to the synthesis of ATP (adenosine triphosphate is a nucleotide that plays an extremely important role in the metabolism of energy and substances in the body). This potential energy can be used by the organism in its further activity. All tissues are the source of heat in the body. Blood, flowing through tissues, heats up. An increase in ambient temperature causes a reflex decrease in metabolism, as a result of which heat generation in the body decreases. With a decrease in ambient temperature, the intensity of metabolic processes reflexively increases and heat generation increases.

The inclusion of chemical thermoregulation occurs when physical thermoregulation is insufficient to maintain a constant body temperature.

Consider these types of thermoregulation.

Physical thermoregulation:

Under physical thermoregulation understand the totality of physiological processes leading to a change in the level of heat transfer. There are the following ways of transferring heat from the body to the environment:

Evaporation (sweating);

Radiation (radiation);

Heat conduction (conduction);

Convection.

Let's consider them in more detail:

1. Evaporation (sweating):

Evaporation (sweating)- this is the return of thermal energy to the environment due to the evaporation of sweat or moisture from the surface of the skin and mucous membranes of the respiratory tract. In humans, sweat is constantly secreted by the sweat glands of the skin (“perceptible”, or glandular, loss of water), the mucous membranes of the respiratory tract are moistened (“imperceptible” loss of water). At the same time, the “perceptible” loss of water by the body has a more significant effect on the total amount of heat given off by evaporation than the “imperceptible” one.

At an ambient temperature of about 20°C, the evaporation of moisture is about 36 g/h. Since 0.58 kcal of thermal energy is spent on the evaporation of 1 g of water in a person, it is easy to calculate that, under these conditions, the body of an adult gives off about 20% of all dissipated heat to the environment through evaporation. An increase in external temperature, performance of physical work, prolonged stay in heat-insulating clothing increase sweating and it can increase up to 500-2.000 g/h.

A person does not tolerate a relatively low ambient temperature (32 ° C) in humid air. In completely dry air, a person can stay without noticeable overheating for 2-3 hours at a temperature of 50-55 ° C. Air-tight clothing (rubber, thick, etc.) that prevents the evaporation of sweat is also poorly tolerated: the layer of air between clothing and the body is quickly saturated with vapor and further evaporation of sweat stops.

The process of heat transfer through evaporation, although it is only one of the methods of thermoregulation, has one exceptional advantage - if the external temperature exceeds the average skin temperature, then the body cannot give off heat to the external environment by other methods of thermoregulation (radiation, convection and conduction), which we will consider below. Under these conditions, the body begins to absorb heat from the outside, and the only way to dissipate heat is to increase the evaporation of moisture from the surface of the body. Such evaporation is possible as long as the humidity of the ambient air remains below 100%. With intense sweating, high humidity and low air velocity, when sweat drops, not having time to evaporate, merge and drain from the surface of the body, heat transfer by evaporation becomes less effective.

When sweat evaporates, our body releases its energy. Actually, thanks to the energy of our body, liquid molecules (ie sweat) break molecular bonds and pass from liquid to gaseous state. Energy is spent on breaking bonds, and as a result, the body temperature drops. The refrigerator works on the same principle. He manages to maintain a temperature inside the chamber, much lower than the ambient temperature. It does this by using electricity. And we do this using the energy obtained from the breakdown of food.

Controlling the selection of clothing can help reduce evaporative heat loss. Clothing should be selected based on weather conditions and current activity. Do not be lazy to take off excess clothes when the loads increase. You will sweat less. And do not be lazy to put it on again when the loads stop. Remove moisture and wind protection if there is no rain with the wind, otherwise the clothes will get wet from the inside, from your sweat. And, in contact with wet clothes, we lose heat also by thermal conductivity. Water conducts heat 25 times better than air. This means that in wet clothes we lose heat 25 times faster. That's why it's important to keep your clothes dry.

Evaporation is divided into 2 types:

a) Imperceptible perspiration(without the participation of sweat glands) is the evaporation of water from the surface of the lungs, mucous membranes of the respiratory tract and water seeping through the epithelium of the skin (evaporation from the skin surface occurs even if the skin is dry).

During the day, up to 400 ml of water evaporates through the respiratory tract, i.e. the body loses up to 232 kcal per day. If necessary, this value can be increased due to thermal shortness of breath. About 240 ml of water seeps through the epidermis on average per day. Therefore, in this way the body loses up to 139 kcal per day. This value, as a rule, does not depend on the processes of regulation and various environmental factors.

b) Perceived perspiration(with active participation of sweat glands) - It is the release of heat through the evaporation of sweat. On average, 400-500 ml of sweat is released per day at a comfortable environmental temperature, therefore, up to 300 kcal of energy is given off. Evaporation of 1 liter of sweat in a person weighing 75 kg can lower body temperature by 10°C. However, if necessary, the volume of sweating can increase up to 12 liters per day, i.е. By sweating, you can lose up to 7,000 kcal per day.

The efficiency of evaporation largely depends on the environment: the higher the temperature and the lower the humidity, the higher the efficiency of perspiration as a heat transfer mechanism. At 100% humidity, evaporation is impossible. At high humidity of atmospheric air, high temperature is more difficult to tolerate than at low humidity. In the air saturated with water vapor (for example, in a bath), sweat is released in large quantities, but does not evaporate and drains from the skin. Such sweating does not contribute to the release of heat: only that part of the sweat that evaporates from the surface of the skin is important for heat transfer (this part of the sweat is effective sweating).

2. Radiation (radiation):

Emission (radiation)- this is a method of heat transfer to the environment by the surface of the human body in the form of electromagnetic waves of the infrared range (a = 5-20 microns). Radiation gives off energy to all objects whose temperature is above absolute zero. Electromagnetic radiation freely passes through a vacuum, atmospheric air can also be considered “transparent” for it.

As you know, any object that is heated above the ambient temperature radiates heat. Everyone felt it while sitting by the fire. The fire radiates heat and heats objects around. In this case, the fire loses its heat.

The human body begins to radiate heat as soon as the ambient temperature drops below the surface temperature of the skin. To prevent heat loss by radiation, exposed areas of the body must be protected. This is done with clothing. Thus, we create a layer of air in clothes between the skin and the environment. The temperature of this layer will be equal to the temperature of the body and the heat loss by radiation will decrease. Why won't the heat loss stop completely? Because now heated clothes will radiate heat, losing it. And, even putting on another layer of clothing, you will not stop the radiation.

The amount of heat dissipated by the body into the environment by radiation is proportional to the surface area of the radiation (surface area of the body not covered by clothing) and the difference between the average temperatures of the skin and the environment. At an ambient temperature of 20°C and a relative air humidity of 40-60%, the body of an adult person dissipates by radiation about 40-50% of the total heat given off. If the ambient temperature exceeds the average skin temperature, the human body, by absorbing infrared rays emitted by surrounding objects, warms up.

Heat transfer by radiation increases with a decrease in ambient temperature and decreases with its increase. Under conditions of constant ambient temperature, radiation from the body surface increases with an increase in skin temperature and decreases with a decrease in it. If the average temperatures of the skin surface and the environment are equalized (the temperature difference becomes equal to zero), then heat transfer by radiation becomes impossible.

It is possible to reduce the heat transfer of the body by radiation by reducing the surface area of radiation - change in body position. For example, when a dog or cat is cold, they curl up into a ball, thereby reducing the heat transfer surface; when it is hot, animals, on the contrary, take a position in which the heat transfer surface increases to the maximum. A person is not deprived of this method of physical thermoregulation, “curling into a ball” while sleeping in a cold room.

3. Heat conduction (conduction):

Heat conduction (conduction)- this is a way of heat transfer, which takes place during contact, contact of the human body with other physical bodies. The amount of heat given off by the body to the environment in this way is proportional to the difference in the average temperatures of the contacting bodies, the area of the contacting surfaces, the time of thermal contact and the thermal conductivity of the contacting body.

Heat loss by conduction occurs when there is direct contact with a cold object. At this moment, our body gives off its heat. The rate of heat loss is highly dependent on the thermal conductivity of the object with which we are in contact. For example, the thermal conductivity of stone is 10 times higher than that of wood. Therefore, sitting on a stone, we will lose heat much faster. You have probably noticed that sitting on a stone is somehow colder than sitting on a log.

Decision? Isolate your body from cold objects using poor heat conductors. Simply put, for example, if you are traveling in the mountains, then settling down on a halt, sit down on a tourist rug or a bundle of clothes. At night, be sure to put a tourist rug under the sleeping bag that matches the weather conditions. Or, in extreme cases, a thick layer of dry grass or needles. The earth conducts (and therefore “takes away”) heat well and cools down a lot at night. In winter, do not pick up metal objects with bare hands. Use gloves. In severe frosts, local frostbite can be obtained from metal objects.

Dry air, adipose tissue are characterized by low thermal conductivity and are heat insulators (poor heat conductors). Clothing reduces heat transfer. Heat loss is prevented by the layer of still air that is between clothing and skin. The heat-insulating properties of clothing are the higher, the finer the cellular structure of its structure, which contains air. This explains the good heat-insulating properties of woolen and fur clothing, which makes it possible for the human body to reduce heat dissipation through heat conduction. The air temperature under the clothes reaches 30°C. Conversely, a naked body loses heat, as the air on its surface is constantly changing. Therefore, the temperature of the skin of the naked parts of the body is much lower than that of the dressed ones.

Humid air saturated with water vapor is characterized by high thermal conductivity. Therefore, a person's stay in an environment with high humidity at low temperature is accompanied by an increase in body heat loss. Wet clothing also loses its insulating properties.

4. Convection:

Convection- this is a method of heat transfer of the body, carried out by transferring heat by moving particles of air (water). Heat dissipation by convection requires air flow around the surface of the body with a temperature lower than that of the skin. At the same time, the layer of air in contact with the skin heats up, reduces its density, rises and is replaced by colder and denser air. Under conditions when the air temperature is 20°C and relative humidity is 40-60%, the body of an adult person dissipates about 25-30% of heat into the environment through heat conduction and convection (basic convection). With an increase in the speed of movement of air flows (wind, ventilation), the intensity of heat transfer (forced convection) also increases significantly.

The essence of the convection process lies in the following- our body heats the air near the skin; heated air becomes lighter than cold air and rises, and it is replaced by cold air, which heats up again, becomes lighter and is displaced by the next portion of cold air. If the heated air is not captured with the help of clothing, then this process will be endless. In fact, it is not clothing that warms us, but the air that it retains.

When the wind blows, the situation worsens. The wind carries huge portions of unheated air. Even when we put on a warm sweater, the wind does nothing to expel warm air from it. The same thing happens when we move. Our body "smashes" into the air, and it flows around us, acting like wind. This also multiplies the heat loss.

What's the solution? Wear a windproof layer: a windbreaker and windproof pants. Don't forget to protect your neck and head. Due to the active blood circulation of the brain, the neck and head are the most heated parts of the body, so the heat loss from them is very large. Also, in cold weather, windy places should be avoided both while driving and when choosing a place to sleep.

Chemical thermoregulation:

Chemical thermoregulation heat generation is carried out due to a change in the level of metabolism (oxidative processes) caused by muscle microvibration (oscillations), which leads to a change in the formation of heat in the body.

The source of heat in the body is the exothermic oxidation reactions of proteins, fats, carbohydrates, as well as the hydrolysis of ATP (adenosine triphosphate is a nucleotide that plays an extremely important role in the metabolism of energy and substances in the body; first of all, this compound is known as a universal source of energy for all biochemical processes occurring in living systems). During the breakdown of nutrients, part of the released energy is accumulated in ATP, part is dissipated in the form of heat (primary heat is 65-70% of energy). When using high-energy bonds of ATP molecules, part of the energy goes to perform useful work, and part is dissipated (secondary heat). Thus, two heat flows - primary and secondary - are heat production.

Chemical thermoregulation is essential for maintaining a constant body temperature both under normal conditions and when the ambient temperature changes. In humans, an increase in heat generation due to an increase in the intensity of metabolism is noted, in particular, when the ambient temperature becomes lower than the optimum temperature, or comfort zone. For a person in ordinary light clothing, this zone is in the range of 18-20°C, and for a naked person it is 28°C.

The optimum temperature during a stay in water is higher than in air. This is due to the fact that water, which has a high heat capacity and thermal conductivity, cools the body 14 times more than air, therefore, in a cool bath, the metabolism increases significantly more than during exposure to air at the same temperature.

The most intense heat generation in the body occurs in the muscles. Even if a person lies motionless, but with tense muscles, the intensity of oxidative processes, and at the same time heat generation, increase by 10%. A small physical activity leads to an increase in heat generation by 50-80%, and heavy muscle work - by 400-500%.

If it is necessary to increase heat production, in addition to the possibility of obtaining heat from the outside, the body uses mechanisms that increase the production of thermal energy. These mechanisms include contractile and nonshivering thermogenesis.

1. Contractile thermogenesis.

This type of thermoregulation works when we are cold and need to raise our body temperature. This method is included in muscle contraction. With muscle contraction, ATP hydrolysis increases, therefore, the flow of secondary heat, which goes to warm the body, increases.

Arbitrary activity of the muscular apparatus, mainly occurs under the influence of the cerebral cortex. At the same time, an increase in heat production is possible by 3-5 times compared with the value of the main exchange.

Usually, when the temperature of the medium and blood temperature decrease, the first reaction is increase in thermoregulatory tone(hair on the body “stands on end”, “goosebumps” appear). From the point of view of the mechanics of contraction, this tone is a microvibration and allows you to increase heat production by 25-40% of the initial level. Usually, the muscles of the neck, head, trunk and limbs take part in creating the tone.

With more significant hypothermia, the thermoregulatory tone turns into a special type of muscle contraction - muscle cold shiver, in which the muscles do not perform useful work and their contraction is aimed solely at generating heat. Cold shivering is an involuntary rhythmic activity of superficially located muscles, as a result of which the metabolic processes of the body are significantly enhanced, the consumption of oxygen and carbohydrates by muscle tissue increases, which entails increase in heat production. Trembling often begins with the muscles of the neck, face. This is due to the fact that, first of all, the temperature of the blood that flows to the brain should rise. It is believed that heat production during cold shivering is 2-3 times higher than during voluntary muscle activity.

The described mechanism works at the reflex level, without the participation of our consciousness. But you can raise the body temperature with the help of conscious motor activity. When performing physical activity of different power, heat production increases by 5-15 times compared to the level of rest. During the first 15-30 minutes of long-term operation, the core temperature rises quite quickly to a relatively stationary level, and then remains at this level or continues to rise slowly.

2. Non-shivering thermogenesis:

This type of thermoregulation can lead to both an increase and a decrease in body temperature. It is carried out by accelerating or slowing down catabolic metabolic processes (oxidation of fatty acids). And this, in turn, will lead to a decrease or increase in heat production. Due to this type of thermogenesis, the level of heat production in a person can increase 3 times compared to the level of basal metabolism.

The regulation of the processes of non-shivering thermogenesis is carried out by activating the sympathetic nervous system, the production of thyroid hormones and the adrenal medulla.

E. Control of thermoregulation.

Hypothalamus.

The thermoregulation system consists of a number of elements with interrelated functions. Information about the temperature comes from thermoreceptors and with the help of the nervous system enters the brain.

Plays a major role in thermoregulation hypothalamus. It contains the main centers of thermoregulation, which coordinate numerous and complex processes that ensure the preservation of body temperature at a constant level.

Hypothalamus- this is a small area in the diencephalon, which includes a large number of groups of cells (over 30 nuclei) that regulate the neuroendocrine activity of the brain and homeostasis (the ability to maintain the constancy of one's internal state) of the body. The hypothalamus is connected by neural pathways to almost all parts of the central nervous system, including the cortex, hippocampus, amygdala, cerebellum, brain stem, and spinal cord. Together with the pituitary gland, the hypothalamus forms the hypothalamic-pituitary system, in which the hypothalamus controls the release of pituitary hormones and is the central link between the nervous and endocrine systems. It secretes hormones and neuropeptides, and regulates functions such as hunger and thirst, body temperature regulation, sexual behavior, sleep and wakefulness (circadian rhythms). Recent studies show that the hypothalamus plays an important role in the regulation of higher functions, such as memory and emotional state, and thus participates in the formation of various aspects of behavior.

Destruction of the centers of the hypothalamus or disruption of nerve connections leads to the loss of the ability to regulate body temperature.

The anterior hypothalamus contains neurons that control heat transfer.(they provide physical thermoregulation - vasoconstriction, sweating). When the neurons of the anterior hypothalamus are destroyed, the body does not tolerate high temperatures, but physiological activity is preserved in cold conditions.

Neurons of the posterior hypothalamus control the processes of heat production(they provide chemical thermoregulation - increased heat generation, muscle tremors). When they are damaged, the ability to increase energy metabolism is impaired, so the body does not tolerate cold well.

The thermosensitive nerve cells of the preoptic region of the hypothalamus directly "measure" the temperature of the arterial blood flowing through the brain and are highly sensitive to temperature changes (they are able to distinguish between a blood temperature difference of 0.011°C). The ratio of cold- and heat-sensitive neurons in the hypothalamus is 1:6, so the central thermoreceptors are predominantly activated when the temperature of the "core" of the human body rises.

Based on the analysis and integration of information about the value of the temperature of the blood and peripheral tissues, the average (integral) value of body temperature is continuously determined in the preoptic region of the hypothalamus. These data are transmitted through intercalary neurons to a group of neurons in the anterior hypothalamus, which set a certain level of body temperature in the body - the "setting point" of thermoregulation. Based on the analysis and comparison of the values of the average body temperature and the set value of the temperature to be regulated, the mechanisms of the "set point" through the effector neurons of the posterior hypothalamus affect the processes of heat transfer or heat production in order to bring the actual and set temperature into line.

Thus, due to the function of the thermoregulation center, a balance is established between heat production and heat transfer, which makes it possible to maintain body temperature within the optimal limits for the life of the body.

Endocrine system.

The hypothalamus controls the processes of heat production and heat transfer by sending nerve impulses to the endocrine glands, mainly the thyroid, and the adrenal glands.

Participation thyroid gland in thermoregulation is due to the fact that the influence of low temperature leads to an increased release of its hormones (thyroxine, triiodothyronine), which accelerate metabolism and, consequently, heat generation.

Role adrenal glands associated with the release of catecholamines (adrenaline, norepinephrine, dopamine) into the blood, which, by increasing or decreasing oxidative processes in tissues (for example, muscle), increase or decrease heat production and constrict or increase skin vessels, changing the level of heat transfer.